Science / Medicine : Shallow Quakes More Destructive

Earthquakes that hit California are quite different from many earthquakes around the world in that California’s temblors are centered only a few miles below the surface of the Earth’s crust and thus are very destructive. In many regions of the globe, earthquakes occur many miles below the surface and cause less shaking--and thus less destruction--than the shallow quakes.

Scientists at UC Santa Cruz think they may have figured out why the two types of quakes are so different, according to their findings, published last week in the journal Nature.

“Very deep earthquakes are different beasts from the shallow ruptures familiar to Californians and others who live near fault zones,” the scientists said.

Shallow earthquakes occur when strain across a fault zone is so great that it causes rocks to break and slip along the fault. However, after studying earthquakes centered about 185 miles below the surface, the scientists concluded that intense pressures at such depths make such fractures impossible. Instead, the area around the epicenter deforms rather than breaking, and it does so very quickly during the quake.

“Somehow, deep events get going faster,” said Heidi Houston, a seismologist. The process is not fully understood, the scientists noted.